Field
The disclosed technology generally relates to material systems which include a plurality of particles, and more particularly to material systems that include composite particles having a core and one or more shells that enclose the core.
Description of the Related Art
Various materials, e.g., metals and metal oxides, can be produced in the form of particles, which can have enhanced properties compared to their bulk counterparts because of, for example, their size and/or surface/volume ratio. For some applications, it may be desirable to at least temporarily prevent the particles from being exposed to external environments. One to way to limit the exposure of the particles to the external environments is to coat the particles with a protective material such as, for example, silicon oxide. Silicon oxide coatings can be formed using a variety of techniques. However, silicon oxide, while effective against some environments, can be ineffective against some other environments. For example, for many biological applications, particles coated with silicon oxide may be subjected to solution environments, e.g., aqueous solutions, in which silicon oxide has an unacceptable dissolution rate (e.g., nanometers within days, depending on the application). Premature dissolution of the protective coating and the resulting exposure of the core particle surface species can be undesirable for many biological applications, since surface-bound targeting molecules can be prematurely released from the particle surface.
To improve the stability of silicon oxide coatings of particles against dissolution in certain environments, e.g., water, a number of techniques can be employed, including, for example, heating in solution, calcining and maintaining the silica in a non-aqueous solution. Heating in solution only partially reduces the solubility of silicon oxide. Calcining, which refers to a process in which silicon oxide is heated to high temperatures (e.g. >200° C.), is effective at condensing Si—OH bonds into Si—O—Si linkages, thus increasing the stability. However, the high temperatures can increase particle agglomeration, making it difficult to redisperse the particles in uniform suspension. The calcination step can also affect other chemical entities within or on the surface of the particles such as the free amines, thiols or carboxylic acids which are utilized for attaching targeting molecules to the surface of the silica particles. While retaining the silica in an organic solvent can prevent dissolution of silicon oxide coating, such methods may not be relevant for many biological applications that require them to be maintained in an aqueous solutions. Thus, there is a need for protective coating of particles that offer greater stability against dissolution in aqueous environments.